| Lithographic printing plates with high print run stability -> Monitor Keywords |
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Lithographic printing plates with high print run stabilityRelated Patent Categories: Radiation Imagery Chemistry: Process, Composition, Or Product Thereof, Imaging Affecting Physical Property Of Radiation Sensitive Material, Or Producing Nonplanar Or Printing Surface - Process, Composition, Or Product, Radiation Sensitive Composition Or Product Or Process Of MakingLithographic printing plates with high print run stability description/claimsThe Patent Description & Claims data below is from USPTO Patent Application 20070184381, Lithographic printing plates with high print run stability. Brief Patent Description - Full Patent Description - Patent Application Claims
[0001] The present invention relates to radiation-sensitive elements having a coating comprising at least one silsesquioxane. The invention furthermore relates to radiation-sensitive compositions used for the production of said elements, a process for imaging such elements and imaged elements obtained therefrom. [0002] The technical field of lithographic printing is based on the immiscibility of oil and water, wherein the oily material or the printing ink is preferably accepted by the image area, and the water or fountain solution is preferably accepted by the non-image area. When an appropriately produced surface is moistened with water and a printing ink is applied, the background or non-image area accepts the water and repels the printing ink, while the image area accepts the printing ink and repels the water. The printing ink in the image area is then transferred to the surface of a material such as paper, fabric and the like, on which the image is to be formed. Generally, however, the printing ink is first transferred to an intermediate material, referred to as blanket, which then in turn transfers the printing ink onto the surface of the material on which the image is to be formed; this technique is referred to as offset lithography. [0003] A frequently used type of lithographic printing plate precursor comprises a photosensitive coating applied onto a substrate on aluminum basis. The coating can react to electromagnetic radiation such that the exposed portion becomes so soluble that it is removed during the developing process. Such a plate is referred to as positive working. On the other hand, a plate is referred to as negative working if the exposed portion of the coating is hardened by the radiation. In both cases, the remaining image area accepts printing ink, or is oleophilic, and the non-image area (background) accepts water, or is hydrophilic. The differentiation between image and non-image areas takes place during exposure, for which a film is attached to the printing plate precursor under vacuum in order to guarantee good contact. The plate is then exposed by means of a radiation source. Alternatively, the plate can also be exposed digitally without a film, e.g. with a UV laser. When a positive plate is used, the area on the film corresponding to the image on the plate is so opaque that the light does not reach the plate, while the area on the film corresponding to the non-image area is clear and allows light to permeate the coating, whose solubility increases. In the case of a negative plate, the opposite takes place: The area on the film corresponding to the image on the plate is clear, while the non-image area is opaque. The coating beneath the clear film area is hardened due to the incident light, while the area not affected by the light is removed during developing. The light-hardened surface of a negative working plate is therefore oleophilic and accepts printing ink, while the non-image area that used to be coated with the coating removed by the developer is desensitized and therefore hydrophilic. [0004] It is known for example from DE 10 113 926 A1, DE 19 739 953 A1 and EP-B-0 900 653 that the use of inorganic particles or polymer particles, such as e.g. SiO.sub.2 or polyethylene particles, in free-radical polymerizable radiation-sensitive coatings of printing plate precursors can improve the print run stability of a plate. [0005] However, processing radiation-sensitive compositions comprising such particles is not very easy since the particles should be distributed as evenly as possible in the coating produced from the composition. Usually, this requires special dispersion processes as well as the use of dispersing agents. Still, an aggregation of the particles in the coating solution frequently occurs which is due to an insufficient storage stability of these solutions. Furthermore, the use of particles causes problems during the filtration of the coating solutions since the pore size of the filter has to be larger than the size of the particles used in the solution. However, insufficiently filtered coating solutions lead to coating imperfections. [0006] Another possibility is the production of coatings containing inorganic particles or polymer particles using the so-called dual feed process (see e.g. U.S. Pat. No. 6,548,215). However, this requires special coating techniques which increases the costs of the coating process. [0007] However, in all cases, the particles present in the coating can easily break out of the coating during printing which then leads to a printed image of unacceptable low quality. [0008] It is therefore the object of the present invention to provide a radiation-sensitive elements which, when used in the production of lithographic printing plates, allows a higher print run of the printing plates with--at the same time--a high degree of radiation sensitivity; furthermore, special dispersion processes should be avoided and break-outs in the image elements during printing should be largely excluded. [0009] This object is achieved by a radiation-sensitive element comprising [0010] (a) an optionally pretreated substrate and [0011] (b) a radiation-sensitive coating consisting of a composition comprising [0012] (i) one or more types of monomers, each comprising at least one ethylenically unsaturated group accessible to a free-radical polymerization, [0013] (ii) at least one radiation-sensitive initiator or initiator system for free-radical polymerization absorbing radiation selected from the wavelength range of 300 to 1,200 nm; and [0014] (iii) at least one silsesquioxane comprising at least one substituent with at least one ethylenically unsaturated group each; [0015] applied on a substrate. [0016] The first essential component of the radiation-sensitive composition of the present invention is a monomer, oligomer and/or polymer with at least one ethylenically unsaturated group accessible to free-radical polymerization. [0017] All monomers, oligomers and polymers which are free-radical polymerizable and comprise at least one C--C double bond can be used as ethylenically unsaturated monomers, oligomers and polymers. Monomers, oligomers and polymers with C--C triple bonds can also be used, but they are not preferred. Suitable compounds are well known to the person skilled in the art and can be used in the present invention without any particular limitations. Esters of acrylic and methacrylic acids, itaconic acid, crotonic acid, isocrotonic acid, maleic acid and fumaric acid with one or more unsaturated groups in the form of monomers, oligomers or prepolymers are preferred. They may be present in solid or liquid form, with solid and highly viscous forms being preferred. Compounds suitable as monomers include for instance trimethylol propane triacrylate and -methacrylate, pentaerythritol triacrylate and -methacrylate, dipentaerythritolmonohydroxy pentaacrylate and -methacrylate, dipentaerythritol hexaacrylate and -methacrylate, pentaerythritol tetraacrylate and -methacrylate, ditrimethylol propane tetraacrylate and -methacrylate, diethyleneglycol diacrylate and -methacrylate, triethyleneglycol diacrylate and -methacrylate or tetraethyleneglycol diacrylate and -methacrylate. Suitable oligomers and/or prepolymers are for example urethane acrylates and -methacrylates, epoxide acrylates and methacrylates, polyester acrylates and methacrylates, polyether acrylates and methacrylates or unsaturated polyester resins. [0018] In addition to monomers and/or oligomers, use can also be made of polymers comprising free-radical polymerizable C--C double bonds in the main or side chains. Examples thereof include reaction products of maleic acid anhydride/olefin copolymers and hydroxyalkyl(meth)acrylates (cf. e.g. DE-A-4311738); (meth)acrylic acid polymers, partially or fully esterified with allyl alcohol (cf. e.g. DE-A-3332640); reaction products of polymeric polyalcohols and isocyanato(meth)acrylates; unsaturated polyesters; (meth)acrylate-terminated polystyrenes, poly(meth)acrylic acid ester, poly(meth)acrylic acids, poly(meth)acrylamides; (meth)acrylic acid polymers, partially or fully esterified with epoxides comprising free-radical polymerizable groups; and polyethers. In this connection, the prefix "(meth)" indicates that both derivatives of acrylic acid and of methacrylic acid can be used. [0019] Additional suitable C--C-unsaturated free-radical polymerizable compounds are described e.g. in EP-A-1 176 007. [0020] It is of course possible to use different kinds of monomers, oligomers or polymers in admixture; furthermore, mixtures of monomers and oligomers and/or polymers can be used in the present invention, as well as mixtures of oligomers and polymers. The free-radical polymerizable monomers/oligomers/polymers are preferably present in an amount of 5 to 95 wt.-%; if monomers/oligomers are used, especially preferred 20 to 85 wt.-%, based on the dry layer weight of a radiation-sensitive coating prepared from the radiation-sensitive composition of the present invention. As used in the present invention, the term "dry layer weight of the radiation-sensitive coating" is therefore synonymous with the term "solids of the radiation-sensitive composition". [0021] Another essential component of the radiation-sensitive composition of the present invention is at least one radiation-sensitive initiator or an initiator system for free-radical polymerization of the monomers, oligomers or polymers present. It depends on the type of initiator or initiator system. whether the radiation-sensitive element of the present invention is imaged with UV radiation, visible (VIS) radiation or IR radiation. [0022] Initiators which directly form free radicals upon absorption of UV radiation are known to the person skilled in the art and are for example described in K.K. Dietliker: "Chemistry & Technology of UV&EB formulation for coatings, inks & prints", Vol. 3 (SITA Technology, London (1991)). They, include for example oxime ethers and oxime esters, benzoins and benzoin ethers, (.alpha.-hydroxy or (.alpha.-aminoacetophenones, acyl phosphine oxides and diacyl phosphine oxides. [0023] Within the framework of the present invention, an initiator system comprising [0024] (a) a sensitizer which absorbs UV, VIS or IR radiation but is unable to form free radicals by itself, and [0025] (b) a coinitiator which by itself, depending on which electromagnetic radiation is used for image-wise exposure, is unable to absorb the UV, VIS or IR radiation emitted from the radiation source, but together with the radiation-absorbing sensitizer used in the present invention forms free radicals is especially preferred. [0026] For the purpose of the present invention, these initiator systems are divided into two groups, namely those wherein the sensitizer absorbs radiation from the range of more than 750 to 1,200 nm (also briefly referred to as IR absorbers) and those wherein the sensitizer absorbs radiation from the range of 300 to 750 nm. When an initiator system of the first group is used, an IR-sensitive element is obtained whereas the use of an initiator system of the second group leads to a UV or VIS-sensitive element. [0027] Based on its absorption properties, the sensitizer used in the initiator systems determines whether the radiation-sensitive composition is sensitive in the UV, VIS or IR range. [0028] In the present invention, the IR absorbers are preferably selected from the class of triarylamine dyes, thiazolium dyes, indolium dyes, oxazolium dyes, cyanine dyes, polyaniline dyes, polypyrrol dyes, polythiophene dyes and phthalocyanine dyes and pigments. However, other IR absorbers, for example those described in EP 1 160 095 A1, can be used as well. [0029] The following compounds are e.g. suitable IR absorbers: [0030] According to one embodiment of the present invention, an IR absorber of formula (I) is used. wherein [0031] each X independently represents S, O, NR or C(alkyl).sub.2; [0032] each R.sup.1 independently represents an alkyl group; [0033] R.sup.2 represents a halogen atom, SR, OR or NR2; [0034] each R.sup.3 independently represents a hydrogen atom, an alkyl group, OR, SR or NR.sub.2 or a halogen atom; R.sup.3 can also be benzofused; [0035] A.sup.- represents an anion; [0036] --- represents an optionally present carbocyclic five- or six-membered ring; [0037] R represents an alkyl or aryl group; in the case of NR.sub.2, one R can also be H; [0038] each n can independently be 0, 1, 2 or 3. [0039] These IR absorbers absorb in the range of 800 to 1,200 nm; those of formula (I) absorbing in the range of 810 to 860 nm are preferred. [0040] X is preferably a group C(alkyl).sub.2, wherein the alkyl group preferably comprises 1 to 3 carbon atoms. [0041] R.sup.1 is preferably an alkyl group with 1 to 4 carbon atoms. [0042] R.sup.2 is preferably SR. [0043] R.sup.3 is preferably a hydrogen atom. [0044] R is preferably a phenyl group. [0045] It is preferred that the dotted line represent the residue of a ring with 5 or 6 carbon atoms. [0046] The counterion A.sup.- is preferably a chloride ion or a tosylate anion. [0047] IR absorbers with a symmetrical structure (I) are especially preferred. [0048] Examples of especially preferred IR absorbers include: [0049] 2-[2-[2-Phenylsulfonyl-3-[2-(1,3-dihydro-1,3,3-trimethyl-2H-indol- e-2-ylidene)-ethylidene]-1-cyclohexene-1-yl]-ethenyl]-1,3,3-trimethyl-3H-i- ndoliumchloride, [0050] 2-[2-[2-thiophenyl-3-[2-(1,3-dihydro-1,3,3-trimethyl-2H-indole-2-ylidene)- -ethylidene]-1-cyclohexene-1-yl]-ethenyl]-1,3,3-trimethyl-3H-indoliumchlor- ide, [0051] 2-[2-[2-thiophenyl-3-[2-(1,3-dihydro-1,3,3-trimethyl-2H-indole-2-ylidene)- -ethylidene]-1-cyclopentene-1-yl]-ethenyl]-1,3,3-trimethyl-3H-indoliumtosy- late, [0052] 2-[2-[2-chloro-3-[2-(1,3-dihydro-1,3,3-trimethyl-2H-benzo[e]-indole-2-yli- dene)-ethylidene]-1-cyclohexene-1-yl]-ethenyl]-1,3,3-trimethyl-1H-benzo[e]- -indolium-tosylate and [0053] 2-[2-[2-chloro-3-[2-ethyl-3H-benzthiazole-2-ylidene)-ethylidene]-1-cycloh- exene-1-yl]-ethenyl]-3-ethyl-benzthiazolium-tosylate. [0054] 1,4-Dihydropyridines, oxazoles, bisoxazoles and analogues, coumarins, triarylmethane dyes and metallocenes can for example be used as UV or VIS-absorbing sensitizers. Such sensitizers are e.g. described in DE 42 17 495 A1, DE 44 18 645 C1, DE 2 801 065 C2, EP 1 041 074 A1 and DD 287 796 A. [0055] In UV- or VIS-sensitive compositions, compounds of the formula (II) can be used as sensitizers of the oxazole type. wherein each R.sup.4, R.sup.5 and R.sup.6 is independently selected from a halogen atom, an optionally substituted alkyl group, an optionally substituted aryl group, which may also be fused, an optionally substituted aralkyl group, a group --NR'R'' and a group --OR''', [0056] wherein R' and R'' are independently selected from a hydrogen atom, an alkyl, aryl or aralkyl group, [0057] R''' is an optionally substituted alkyl, aryl or aralkyl group or a hydrogen atom, and k, m and n are independently 0 or an integer from 1 to 5. 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